Pegasus puts into orbit the first of a new class of small satellites

The first satellites were tiny, antenna-studded devices that often weighed little more than the men and women who built them. But big was better as the space age progressed. The largest satellites today tip the scales at 15 tons, cost hundreds of millions of dollars and are roughly the size of Mack trucks. They must be put into orbit by giant rockets or space shuttles.

But in the skies over California last week, a launch took place that broke all the rules. A diminutive rocket named Pegasus, built by a Virginia-based entrepreneurial firm called Orbital Sciences, dropped from under the wing...